The present invention relates generally to a method for generating hydrogen by decomposing water.
Hydrogen has been hitherto produced through the electrolysis of water. This producing process does necessitate valuable electric energy in order to electrolyze water. Concurrently, the research is developed for techniques producing hydrogen with semiconductor particulates. In the process using semiconductor particulates, as shown in FIG. 1, an element Pd of platinum group metal is carried on a semiconductor particulate 101 such as TiO.sub.2, which is then introduced into water 102. Exposure of the water 102 to sunlight 103 causes electrons 105 within a valence band 104 to excite over an energy gap to a conduction band 106, thereby generating positive holes 107 in the valence band 104. The electrons excited to the conduction band 106 reduce protons H.sup.+ in the water 102 to hydrogen atoms H. Two hydrogen atoms join together to make a hydrogen molecule H.sub.2. The positive holes 107 remaining in the valence band 104 deprive water H.sub.2 O or hydroxide ion OH.sup.- of electrons, generating oxygen or protons. Organic substances in water, if present, are also oxidized. The element Pd of platinum group metal functions to promote oxidation/reduction reaction.
The above-described process, however, possibly produces a very small amount of hydrogen irrespective of a vast sun exposure area because of its lower sunlight transmittance and lower light energy harnessing efficiency. Further, this hydrogen producing process cannot work during night or in rain. Under such circumstances, an alternative process has been sought for producing hydrogen.
For instance, Japanese Patent Laid-open Pub. No. Hei 2-595440 (1990) discloses a hydrogen producing process producing hydrogen using a catalyst such as a radioactive element of platinum group metal generated in a nuclear reactor. In this hydrogen producing process, as shown in FIG. 2B, a radioactive element 110 of platinum group metal such as Ru-106 is carried on a semiconductor particulate 111. When the semiconductor particulate 111 is introduced into water, radiation h.mu. originating from the radioactive element 110 of platinum group metal causes electrons 113 within a valence band 112 to excite over the energy gap. The excitation of the electrons 113 makes it possible to produce hydrogen in the same manner as the above-described process. The hydrogen producing process utilizing a radiation has the following advantages: (a) capable of realizing mass production of electron/positive hole pairs; (b) allowing a substance of a larger band gap to be the semiconductor particulate; (c) capable of increasing the particulate concentration in a water/catalyst medium; and (d) providing a way of utilizing a radioactive waste of a higher radioactivity level.
The hydrogen producing process in which a radioactive element of platinum group metal is carried on a semiconductor particulate, however, requires crushing and mixing the element of platinum group metal when carrying the element on the semiconductor, resulting in additional steps dealing with a high-radioactive substance which is difficult to handle. In addition, the radioactive substances have to be selected from radioactive elements of platinum group metal, since the substances used in this process is also required to act as a catalyst for oxidation/reduction reaction. This fact makes it impossible to provide the other radioactive substances with useful application.